1. Field of the Invention
The present invention provides an economical and efficient method of reworking printed circuit boards (PCB) having either individual semiconductor devices (chips or ICs) or modules, including single chip or multichip, soldered thereon. More specifically, a non-stick or reworkable coating is applied intermediate the chip and PCB such that encapsulant surrounding the chip is unable to adhere to the PCB and can be removed subsequent to reflow of the solder.
2. Description of Related Art
Encapsulation of chips and multichip modules using a polymer type material is currently known in the art. One reason for encapsulating integrated circuit devices is to seal the input/output points of the chips from external contaminants. Additionally, a rigid polymer material will provide support to the chip or module relative to the substrate, or PWB that is the carrier. Generally, chips and MCMs have a different coefficient of thermal expansion when compared to the carrier on which they are affixed. Therefore, these materials will not exhibit the same characteristics when they are heated to operating temperatures inside a computer system. Thus, mismatch in the thermal coefficient of expansion (TCE) normally exists between chips and/or MCMs and the carrier substrate on which they are placed. This inconsistent TCE means that the chip and/or MCM will expand at a different rate per degree increase in temperature than the carrier, thereby causing stress on the I/Os connecting the chips and carrier. Conventional solutions to this problem include placing an encapsulant material between the chip and/or carrier which mechanically bonds the chip to the carrier, which relieves some of the stress caused by the expansion.
Those skilled in the art will understand that a substantial number of chips, MCMs, and other electronic components that are placed on substrate carriers for use in computer systems, or the like, do not pass the test phase of manufacturing. In these cases the chips must be removed from the carrier and replaced with other ICs that were tested as good. This replacement is commonly referred to as "rework" wherein the bad chips are replaced with good ones and the system is tested again. It is cost effective and efficient, from a manufacturing standpoint, to test the chips and/or MCMs prior to their encapsulation to avoid rework whenever possible, since it is a costly and time consuming process. However, even with testing a substantial number of components must still be reworked. Therefore, minimizing the amount of time and effort required to rework a carrier is a serious consideration to the computer industry.
Conventional systems place a single material intermediate of the chip and carrier. For example, U.S. Pat. No. 5,086,558 places an interposer between a chip and substrate, and uses relatively weak adhesive to bond the interposer to the substrate and a stronger adhesive to bond to the chip. The interposer is a thermoplastic material and will remain adhered to the chip when reworked, due to the weak adhesive used to bond it to the
IBM Technical Disclosure Bulletin, volume 34, No. 2, July 1991, pages 181-182 describes a chip that is wirebonded to a carrier and encapsulated to eliminate wire breakage. To assist rework, a release layer is provided between the circuit board and the epoxy encapsulant. This release layer is not placed intermediate the chip and board.
U.S. Pat. No. 4,604,644 shows a chip connection scheme wherein an organic material is disposed between and bonded to the chip and substrate. This bonded material is used to reduce stresses exerted on the module, due to mismatched thermal coefficients of expansion. Additionally, the material is only required to be placed underneath a portion of the periphery of the chip, and is not placed near the center of the chip/substrate interface.
U.S. Pat. Nos. 5,089,440 and 5,121,190 each describe a material that is placed between a chip and substrate to reduce thermal mismatch.
U.S. Pat. No. 4,582,556 discusses encapsulation of a copper, or copper alloy lead frame. A mold release agent is included in the encapsulant material which allows the hardened encapsulant material to release from a mold which is forming the lead frame.
It can be seen that none of the prior art shows a material that allows fast and efficient rework of modules having encapsulant material intermediate the chip and substrate. Therefore, it is desirable to have a system and method that will allow cost effective and efficient rework of multichip modules.